Integrated circuit fabrication techniques vary greatly depending on the specific chip structure being made, the exact processes being used, and/or the available equipment.
However, almost all fabrication methods include a lithography process during which certain regions of a wafer (i e., a silicon slice coated with a photoresist material) are exposed to radiation to delineate a latent image corresponding to the desired circuit pattern. The radiation-exposed wafer is then developed, etched, and processed to form an integrated circuit.
The technical advances in lithography processes have been significant. Integrated circuits built to design rules at or slightly below 0.25 .mu.m are common with the use of radiation in the deep ultraviolet wavelength. Radiation in the extreme ultraviolet (EUV) range (3 nm to 50 nm wavelength--also referred to as "soft x-ray") has been found useful for the fabrication of devices having design rules of 0.18 .mu.m and is prospectively useful for even smaller design rules, such as 0.10 .mu.m and smaller.
During the past eight years, EUV lithography has evolved from a simple concept into a possible candidate for mass commercial production of integrated circuits. Projection lithography, and particularly reflective (rather than transmission) projection lithography, is believed to be the best route to industrial production of integrated circuits by use of EUV lithography. In such a system, EUV radiation is projected onto a lithography mask having reflective regions and non-reflective regions corresponding to the desired circuit pattern. The beams reflected from the mask are then demagnified and projected onto the wafer.
Of particular interest in the present invention is the reflective mask used in EUV projection lithography. A reflective EUV lithography mask typically comprises a substrate, a reflective coating on a top surface of the substrate, and a plurality of absorbing blocks covering certain regions of the reflective coating in a manner corresponding to a desired circuit pattern. A reflective mask may also include buffer blocks situated between the covered regions of the reflective coating and the absorbing blocks.
A method of making a reflective lithography mask typically comprises the steps of applying the reflective coating onto the substrate and then applying a buffer layer on the reflective coating to create a reticle blank. The absorber blocks are then arranged on the buffer layer in a manner corresponding to a desired circuit pattern by, for example, depositing an absorber layer on the buffer layer and etching certain portions of the absorber layer to form the absorber blocks. The uncovered regions of the buffer layer are then removed, usually by etching, to create a plurality of buffer blocks situated between block-covered regions of the reflective coating and the absorbing blocks. Accordingly, the buffer material must remain intact during creation of the absorbing blocks while at the same time must be able to be removed from the uncovered regions of the reflective coating without damaging the coating.
Reflective masks may have a tendency to accumulate and retain static electric charge when exposed to intense actinic radiation, such as during the EUV lithography session. At the very least, this static charge attracts dust which may interfere with the exposure of the mask. Moreover, perhaps more importantly, an electrostatic discharge ("ESD") will occur when the electrostatic charge becomes substantial enough to overcome a dielectric material between the charge and another surface of lower electrical potential. Electrostatic discharge can cause permanent or costly damage to an already expensive EUV reflective mask.
Accordingly, the inventor appreciated that a need remains for a reflective lithography mask which is designed to reduce the risk of electrostatic discharge damage.